linux-kernel - Re: [PATCH v2 03/12] x86/sev: Add an x86 version of prot_guest

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Date:   Tue, 24 Aug 2021 08:14:34 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        iommu@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-graphics-maintainer@...are.com,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        kexec@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
        Borislav Petkov <bp@...en8.de>,
        Brijesh Singh <brijesh.singh@....com>,
        Joerg Roedel <joro@...tes.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH v2 03/12] x86/sev: Add an x86 version of prot_guest_has()

On Thu, Aug 19, 2021 at 01:33:09PM -0500, Tom Lendacky wrote:
> I did it as inline originally because the presence of the function will be
> decided based on the ARCH_HAS_PROTECTED_GUEST config. For now, that is
> only selected by the AMD memory encryption support, so if I went out of
> line I could put in mem_encrypt.c. But with TDX wanting to also use it, it
> would have to be in an always built file with some #ifdefs or in its own
> file that is conditionally built based on the ARCH_HAS_PROTECTED_GUEST
> setting (they've already tried building with ARCH_HAS_PROTECTED_GUEST=y
> and AMD_MEM_ENCRYPT not set).
> 
> To take it out of line, I'm leaning towards the latter, creating a new
> file that is built based on the ARCH_HAS_PROTECTED_GUEST setting.

Yes.  In general everytime architectures have to provide the prototype
and not just the implementation of something we end up with a giant mess
sooner or later.  In a few cases that is still warranted due to
performance concerns, but i don't think that is the case here.

> 
> > 
> >> +/* 0x800 - 0x8ff reserved for AMD */
> >> +#define PATTR_SME			0x800
> >> +#define PATTR_SEV			0x801
> >> +#define PATTR_SEV_ES			0x802
> > 
> > Why do we need reservations for a purely in-kernel namespace?
> > 
> > And why are you overoading a brand new generic API with weird details
> > of a specific implementation like this?
> 
> There was some talk about this on the mailing list where TDX and SEV may
> need to be differentiated, so we wanted to reserve a range of values per
> technology. I guess I can remove them until they are actually needed.

In that case add a flag for the differing behavior.  And only add them
when actually needed.  And either way there is absolutely no need to
reserve ranges.